OPC UA (OPC Unified Architecture) is an industrial communication protocol of the OPC Foundation for vendor-independent exchange of machine data, particularly in process automation. OPC UA is a relatively new standard in which the original focus was not on the control of an industrial plant, but rather on standardized information exchange, in particular standardized information exchange between devices from different vendors.
OPC UA has now also been integrated directly into automation technology devices, thereby creating a need for consistent writing of their data. Automation technology plants need to exchange process information, such as process values, measurement values, parameters, control commands, between different devices. It is important that information be transmitted consistently and resiliently between them there. This is particularly important in the case of the devices' data-modifying calls (i.e. the writing of variables).
The OPC UA protocol is also capable of communicating via Internet connections. In the Internet, the prevailing arrangement requires that a client/server communication always be initiated by the client, i.e. only the client is permitted to send requests and the server may then reply to the client's request. However, this conflicts with automation plants' requirement that communication must be initiated by either of the communication partners at any time.
The OPC UA protocol currently solves this problem by already sending at least one request from the client to the server, even in advance of actually requesting data communication from the server, as is shown in FIG. 2. When the server (UA-S) wishes to send data to the client (UA-C), it can do this in accordance with the protocol as a response to one of these requests. These advance requests (PRI, PR2, PR3) are referred to as “Publish Requests” and the associated responses (PA1, PA2, PA3) as “Publish Responses”. Up to three requests are normally retained. When the server has had no need to send data to the client for a long time, it sends an “empty” response, also referred to as a “Keep Alive” response. When the client receives a “Publish Response” to its “Publish Request”, it sends a new request to the server.
This “Keep Alive” mechanism is necessary since the data traffic on the Internet is monitored by firewalls/proxies and similar network elements. These network elements clear down a connection when no data traffic has flowed via it for some time. Furthermore, this mechanism is also used to inform the communication partners of the correct function of the connection. With current means, a Keep Alive message must be dispatched every 10 seconds in order to keep the transmitter and receiver synchronized.
The spread of OPC UA and the use of cellphones and tablets for maintenance purposes are expected to only increase in future. Mobile devices communicate wirelessly using radio signals, such as wireless LAN, Bluetooth, 3G, and LTE, for example.
Since the mobile devices are normally powered by means of a battery and the energy requirements of the applications running on the device directly impact the running time of the device, their operation must be as energy-efficient as possible. The greatest energy consumption in mobile devices occurs in the transmission of messages. The transmitting antenna is therefore activated as little as possible. Reception, on the other hand, does not have a critical effect on the energy balance.
When OPC UA communication is used in a mobile device (e.g. as mobile access to part of a plant for the purposes of maintenance, monitoring, parameterization, etc.), transmission is frequently implemented by means of the OPC UA's own request-response based communication behavior. Since the mobile device is rarely able to switch off its transmitter, the battery of the device is very quickly discharged by this behavior. This poses a problem, particularly when the devices are required to respond promptly to infrequent warnings or events.
The situation of mobile communication was not taken into account in the definition of the OPC UA standard. Mobile terminals such as MOBIC, the MOBile Industrial Communicator, an industry-enabled mobile Internet Pad from Siemens for local and worldwide access to the Intranet and Internet using plug-in radio cards and wireless LAN, which is already used in plants today, communicate using other protocols.
Today, there is still no established solution to this problem. The spread of the use of OPC UA and the increasing use of mobile terminals for maintenance purposes are expected in future.